Field of the Invention
The present invention relates to the conversion of the state of polarization of an optical beam.
Description of the Related Art
The state of polarization of an optical beam represents an important feature in many applications, in particular when measurements comprise polarization dependent components. Polarizers are typically inserted into the optical beam in order to provide a defined state of polarization. While optical signals with defined states of polarization will pass the polarizer, optical signals with other states of polarization will generally be absorbed or reflected. Disadvantageous in that solution, however, is that the optical power of the output beam after the polarizer can be significantly decreased with respect to the input beam. Further, the optical power of the output beam becomes a function of the state of polarization of the input beam.
Malus' law originates from the French scientist and mathematician Étienne-Louis Malus (23 Jul. 1775-24 Feb. 1812). The law states that the transmission of polarized light through a polarizer varies as the square of the cosine of the angle of the polarizer with respect to the light input polarization vector.I=I0 cos2 θi  (1)where I0 is the initial intensity, and θi is the angle between the light's initial polarization direction and the axis of the polarizer. Unpolarized light consists of the superposition of all possible polarization states. The average transmission of unpolarized light through a polarizer is therefore the average of all possible cosine squared values, which equals one half, or 50% transmission, thus the transmission coefficient becomes
                              I                      I            0                          =                              1            2                    .                                    (        2        )            
In practice, some light is lost in the polarizer and the actual transmission of unpolarized light will be somewhat lower than this, around 38% for Polaroid-type polarizers but considerably higher (>49.9%) for some birefringent prism types. If two polarizers are placed one after another (the second polarizer is generally called an analyzer), the mutual angle between their polarizing axes gives the value of θ in Malus' law. If the two axes are orthogonal, the polarizers are crossed and in theory no light is transmitted, though again practically speaking no polarizer is perfect and the transmission is not exactly zero.
Accordingly, there is a need in the art for a polarizer that improves the optical power of the output beam.